A Belarusian artist who once dumped manure outside an office of President Alexander Lukashenko has died in a prison, where he was serving a five-year sentence, human rights activists and his wife said Tuesday.

Ales Pushkin, 57, died in a prison in Grodno in western Belarus of an unknown cause, even though he wasn't known to be sick, according to the Viasna human rights center.

His wife, Janina Demuch, told The Associated Press he "died in the intensive care unit of the prison under unclear circumstances."

Belarusian authorities did not comment.

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Pushkin was a political performer and cartoonist whose subject was often Lukashenko, the country's authoritarian leader. The artist painted Lukashenko in hell, surrounded by riot police, on a fresco in a church in the Belarusian city of Bobr.

In 1999, Pushkin was sentenced to two years for "Dung for the President," in which he overturned a dung cart at the entrance to the presidential office in Minsk, the country's capital.

Belarussian President Alexander Lukashenko

A man who once dumped manure outside the office of President Alexander Lukashenko, above, has died during his five-year prison sentence, according to his wife. (Contributor/Getty Images)

Pushkin was an active participant in political opposition protests. In March 2021, he was arrested, then sentenced to five years in prison for inciting hatred and "desecration of state symbols." Allegedly, at one of his exhibitions, Pushkin painted a Belarusian nationalist who collaborated with the Nazis during World War II. During the sentencing, Ales undressed in protest, for which he was placed in solitary confinement for 13 days.

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Belarus was gripped by massive protests when Lukashenko was re-elected in August 2020 in balloting that some citizens and Western officials considered fraudulent. Authorities responded with a brutal crackdown that resulted in the arrest of more than 35,000 people, police beatings and the shutdown of many non-governmental organizations and independent media outlets.

According to Viasna, Belarus has jailed nearly 1,500 political prisoners, including Nobel Peace Prize winner Ales Bialiatski.

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Exiled Belarusian opposition leader Sviatlana Tsikhanouskaya said she was "heartbroken" about Pushkin's death, and demanded an investigation.

"It is clear that Pushkin has become another tragic victim of the Lukashenko regime," she said. "Thousands of political prisoners are suffering in Belarusian prisons for taking part in pro-democracy protests, supporting Ukraine, or simply expressing their beliefs."